They will lay more often if they are submerged, but they will also lay some times when on land. The only thing is that when they are not floating, they are laying eggs inside their body and the only way for you to see them is to K mode and you need to move them to pick the eggs. However, if you park them on a ceiling, and demo that ceiling, they will float in the air and the eggs will fall on the ground for you to pikcup. ?